The present invention relates to a heater for the pyrolysis of hydrocarbons and particularly to a heater for the steam cracking of paraffins to produce olefins. In particular, the invention relates to a firing arrangement to prevent flame rollover and impingement on the process coils most particularly for staged combustion for low NOx production.
The steam cracking or pyrolysis of hydrocarbons for the production of olefins is almost exclusively carried out in tubular coils located in fired heaters. The pyrolysis process is considered to be the heart of an olefin plant and has a significant influence on the economics of the overall plant.
The hydrocarbon feedstock may be any one of the wide variety of typical cracking feedstocks such as methane, ethane, propane, butane, mixtures of these gases, naphthas, gas oils, etc. The product stream contains a variety of components the concentration of which are dependent in part upon the feed selected. In the conventional pyrolysis process, vaporized feedstock is fed together with dilution steam to a tubular reactor located within the fired heater. The quantity of dilution steam required is dependent upon the feedstock selected; lighter feedstocks such as ethane require lower steam (0.2 lb./lb. feed), while heavier feedstocks such as naphtha and gas oils require steam/feed ratios of 0.5 to 1.0. The dilution steam has the dual function of lowering the partial pressure of the hydrocarbon and reducing the carburization rate of the pyrolysis coils.
In a typical pyrolysis process, the steam/feed mixture is preheated to a temperature just below the onset of the cracking reaction, typically 650° C. This preheat occurs in the convection section of the heater. The mix then passes to the radiant section where the pyrolysis reactions occur. Generally the residence time in the pyrolysis coil is in the range of 0.2 to 0.4 seconds and outlet temperatures for the reaction are on the order of 700° to 900° C. The reactions that result in the transformation of saturated hydrocarbons to olefins are highly endothermic thus requiring high levels of heat input. This heat input must occur at the elevated reaction temperatures. It is generally recognized in the industry that for most feedstocks, and especially for heavier feedstocks such as naphtha, shorter residence times will lead to higher selectivity to ethylene and propylene since secondary degradation reactions will be reduced. Further it is recognized that the lower the partial pressure of the hydrocarbon within the reaction environment, the higher the selectivity.
The flue gas temperatures in the radiant section of the fired heater are typically above 1,100° C. In a conventional design, approximately 32% to 40% of the heat fired as fuel into the heater is transferred into the coils in the radiant section. The balance of the heat is recovered in the convection section either as feed preheat or as steam generation. Given the limitation of small tube volume to achieve short residence times and the high temperatures of the process, heat transfer into the reaction tube is difficult. High heat fluxes are used and the operating tube metal temperatures are close to the mechanical limits for even exotic metallurgies. In most cases, the allowable maximum tube metal temperatures limit the extent to which residence time can be reduced as a result of a combination of higher process temperatures required at the coil outlet and the reduced tube length (hence tube surface area) which results in higher flux and thus higher tube metal temperatures. The exotic metal reaction tubes located in the radiant section of the cracking heater represent a substantial portion of the cost of the heater so it is important that they be utilized fully. Utilization is defined as operating at as high and as uniform a heat flux and metal temperature as possible consistent with the design objectives of the heater. This will minimize the number and length of the tubes and the resulting total metal required for a given pyrolysis capacity.
In the design of ethylene cracking heaters, the process coils are suspended between two planes of firing. In the majority of cracking furnaces, at least a portion of the heat is supplied by hearth or floor burners that are installed on the floor of the firebox. Fuel and air are injected vertically into the firebox from the burners up along the walls and combustion occurs within the firebox in an essentially vertical direction up the walls. In a properly designed system, all of the combustion takes place in this vertical direction against the wall. The balance of the heat is supplied by burners located in the vertical walls and designed to fire radially along the vertical wall.
Typically a plurality of both hearth (floor) burners and wall burners are used to heat the wall which re-radiates that heat to the process coil. The flow of combusting gases in these heaters is essentially vertically up along the wall. This vertical flow results in a recirculation zone in which at some height above the hearth, the gas moves toward the coil plane, flows in a downward direction along the coil plane and then re-enters the vertical burner air flow. This recirculation pattern satisfies the momentum balance at the burners.
While combustion is taking place within this vertical flow of gases, it is desired to keep the combusting gases or flames against the wall and complete the combustion prior to reaching the top of the recirculation zone. This avoids “flame rollover” where the flame turns inwardly toward the centrally located vertical process coil tube-bank through which the process fluid flows. A flame is defined as a flow of gases that are still undergoing combustion reactions and is distinct from the hot gases wherein the combustion has been completed. While combustion is taking place, the combusting gases have higher temperatures. This heat is transferred to the fully combusted gases (flue gases) also within the box and ultimately to the process coils. If a “flame” contacts the process coil, higher than desired heat flux to the tubes and higher than desired tube metal temperatures can result. This in turn will lead to higher rates of coking (over-reaction) inside the tube at that point and limit the run-length or it will lead to carburization of the coil and mechanical failure at that point. Either way, it is not a desirable result. Therefore, the burners must be designed such that the combustion is finished prior to reaching the top of the recirculation or vortex zone.
In general, prior art burners were able to keep the combustion against the wall by imparting a vertical velocity to the airflow and initiating the combustion inside the burner throat. This created a vertical acceleration that allowed the combustion to be completed prior to the flame rolling over toward the coil plane and into the recirculation pattern. However, that was not always true and is not generally true for the new lower NOx combustion type burners. In these low NOx burners, the combustion is staged and purposely moved outside the main burner throat area. The main burners are fired with all of the air required but with a reduced or lean fuel flow. The additional fuel required is then injected separately into the burning mixture. This staged or delayed combustion results in lower maximum flame temperatures and reduced NOx production. There is also less intense vertical momentum being imparted by the staged combustion. In many cases as a result of fuel staging, the combustion is not completed by the top of the recirculation zone. Further, as a result of the lower vertical momentum, the recirculation zone is located lower in the heater. Thus the combination of slower combustion and lower recirculation zone height lead to flame rollover and the severe negative consequences on the process coil.